Gasification of residues, and especially of heavy oil based products (e.g., petroleum coke, visbreaker bottoms, asphaltenes, vacuum bottoms, etc.), is often accompanied by generation of significant quantities of metal carbonyls. For example, nickel and/or iron carbonyls are typically formed in gasification of vacuum bottoms. Metal carbonyls are highly undesirable as they are not only toxic and carcinogenic at relatively low quantities, but also plate in various portions of a combustion turbine.
To avoid such problems, numerous approaches have been developed to at least partially remove metal carbonyls from various gas streams. For example, surfaces in contact with a gas stream containing the metal carbonyls may be coated with austenitic (18/8) stainless steel to avoid reaction with the metal carbonyls. While such a coating may reduce metal plating on the so treated surfaces to at least some degree, use of stainless steel is relatively expensive. Furthermore, coating of surfaces susceptible to metal plating with stainless steel will not (at least to a significant degree) reduce the concentration of metal carbonyls in the gas stream and therefore only shift the problems associated with metal carbonyls to a location downstream of the stainless steel coating.
In another approach, Dvorak et al. employed spent catalysts comprising Cu and/or CuO and ZnO to reduce the concentration of sulfur compounds and iron carbonyl in a gas stream (Chemical Abstracts, Vol. 96 (1982), Abstract No. 164.903e). While the spent catalysts were relatively effective for removal of sulfur compounds, only small amounts of iron carbonyl were removed from the gases. Moreover, Cu and CuO sorbents are known to exhibit significant activity as hydrogenation catalysts. Consequently, when such catalysts are used in syngas, conversion of at least a portion of the syngas to methane and alcohols is almost unavoidable.
To improve removal of iron carbonyl from a gas stream, the gas stream may be contacted with ZnO and/or ZnS as proposed in EP023911A2. In such systems, ZnO and/or ZnS reduced the concentration of iron carbonyl to a significant extent (e.g., 99%), however, nickel carbonyl was removed in this system to a considerably lower degree (e.g., 77%).
In yet another approach, zeolites have been employed to reduce metal carbonyls from gas streams (Golden et al. Sep. Sci. and Techn. (1991), 26, 12: 1559-1574). While zeolites typically reduce the concentration of metal carbonyls from a syngas with relatively high efficiency, the zeolites system described by Golden et al was limited to gas streams that are substantially free of hydrogen sulfide.
In a still further approach, as described in U.S. Pat. No. 5,451,384 to Carr, a gas stream containing metal carbonyls is contacted with lead oxide that is bound on a solid support (e.g., alumina). Lead oxide-based removal of metal carbonyls, and particularly iron carbonyl, is relatively effective, however, has various significant disadvantages. Among other things, the gas stream typically needs to be free of appreciable quantities of sulfur compounds to avoid sorbent poisoning. Furthermore, a highly toxic lead nitrate solution is employed to coat the carrier via a calcination process, which poses environmental and health hazards. Moreover, operation of lead oxide beads at temperatures higher than 100° C. will tend to produce carbon deposits, especially in the absence of hydrogen.
To circumvent at least some of the problems associated with lead oxide, a hydrophobic porous adsorbent may be employed as described in U.S. Pat. No. 6,165,428 to Eijkhout et al. Suitable adsorbents include Si/Al-containing zeolites with a pore size of between about 0.5 nm to 4.0 nm and an average pore volume of 0.005 ml/g sorbent. Among various other advantages, Eijkhout's system can operate under conditions where the gas stream comprises significant amounts of hydrogen sulfide and water. However, effective removal of metal carbonyls is at least in part dependent on proper pore size as Si/Al-containing zeolites are thought to act as molecular sieves. Consequently, disposal of saturated Si/Al-containing zeolites will still pose substantial health and environmental risks due to the high toxicity and low boiling point of metal carbonyls.
Further known adsorption methods for metal carbonyls include those described in U.S. Pat. No. 3,466,340 in which iron carbonyl is removed from liquid methanol or other alcohols using a solid ion exchange resin containing amino groups. Similarly, in French Pat. No. 2,040,232, iron carbonyl-contaminated methanol is passed through a bed of Fe2O3 pellets to remove the iron carbonyl.
In U.S. Pat. No. 4,608,239, the inventors describe iron carbonyl removal from a gas using alkali metal hydroxide in association with a high boiling hydroxylic solvent to form nonvolatile iron carbonylate salts, which are then separated from the gas. Alternatively, as described in U.S. Pat. No. 3,780,163, ozone is reacted with iron carbonyl from a gas containing carbon monoxide or from a liquid (e.g., ethyl acetate). However, all, or almost all of such known processes either result in a relatively toxic product that needs to be disposed of, or use highly toxic reagents that need to be destroyed or otherwise removed where such reagents are employed in molar excess to the metal carbonyl.
Therefore, although various configurations and processes are known in the art to remove metal carbonyls from a gas stream, all or almost all suffer from one or more disadvantages. Thus, there is still a need for improved configurations and processes for carbonyl removal.